I could try to explain the different types of string to you but another stringer "Colin the Stringer" has has already done a much better job than I could so I will just send you there. Btw. "Colin" calls Poly strings monofilaments. String Types
He also has an an excellent list of Strings the Pros use
Some advice from other professionals on racquet stringing.
Choosing a string 1.
Choosing a string 2.
String (guage) theory - long-winded but all good stuff.
How natural gut is made - not essential information but interesting and you will perhaps see why natural gut is soooo expensive.